In "--trace" output, print elapsed miliseconds for ballparking performance issues
We can reasonably turn --trace into a rough-cut profiler by adding milliseconds-since-start. Then --trace output is good enough to at least roughly ballpark where slowness is happening in many cases. This isn't as good as real profile and won't always be helpful, but at least some of the time it can at least give us enough information to know if, e.g., we should break out the profiler or not.
Test Plan: Ran bin/harbormaster rebuild-log --id ... --trace --force, got milliseconds-since-start in output and some vague idea about where time was being spent.
Reviewed By: amckinley
Maniphest Tasks: T13164
Differential Revision: https://secure.phabricator.com/D19565